Solid oxide fuel cell ("SOFC") systems convert chemical energy from reactant gases into electrical energy. Fuel gases such as H.sub.2, CO, CH.sub.4 or other hydrocarbon-containing gases combust with oxygen from air in a high-temperature electrochemical reaction occurring across ion-specific, solid oxide electrolyte plates. Electrolyte materials that are able to withstand the high-temperature process and are particularly suited for use in SOFC systems include ZrO.sub.2, CeO.sub.2 and Bi.sub.2 O.sub.3. The electrolyte plates have an oxygen electrode (cathode) positioned on one face, and a fuel electrode (anode) positioned on the opposite face. Air is supplied to the oxygen electrode, and fuel gas is supplied to the fuel electrode. Oxygen from the air reacts at the cathode according to the equation: EQU O.sub.2 +4e.sup.- .fwdarw.2O.sup.-2
The oxygen anions produced at the cathode migrate through the ion specific electrolyte plate to the anode which is exposed to fuel gases. The reaction at the anode is shown in equations: EQU H.sub.2 +O.sup.-2 .fwdarw.H.sub.2 O+2e.sup.-
and EQU CO+O.sup.-2 .fwdarw.CO.sub.2 +2e.sup.-
An electric current may be obtained by electrically connecting the anode and cathode due to the current flux of electrons involved in the electrochemical reaction. The overall reaction is shown in the equations: EQU O.sub.2 +2H.sub.2 .fwdarw.2H.sub.2 O EQU O.sub.2 +2CO.fwdarw.2CO.sub.2
Typical SOFCs include a plurality of electrolyte plates arranged into stacks where adjacent plates in a stack are separated by interconnectors that permit reactant gases to flow between the plates and contact the electrodes. The planar interconnectors are sized and shaped to resemble the planar electrolyte plates, but have opposing grooved surfaces; the grooves on one face are typically oriented 90.degree. or perpendicular to the grooves on the opposite face. This arrangement (referred to as a cross-flow geometry) permits the flow of fuel gases through one set of grooves, while air flows through the other set of grooves oriented on the 90.degree. orthogonal axis. In this manner, fuel and air flow through the electrolyte plates of the fuel cell stack at 90.degree. angles. The interconnectors contact the electrolyte plates around their perimeter and along the ridges of the grooves. Reactant gases flowing through the grooves are in fluid communication with the electrodes on the surface of the electrolyte plates.
A gas-tight seal between the interconnectors and the electrolyte plates, particularly around the perimeter is very important to prevent the gases flowing on opposite sides of the interconnector from mixing. At the high operating temperature of a SOFC (800.degree.-1000.degree. C.), if air is permitted to mix with the fuel stream, the electrochemical reaction intended to occur across the electrolyte plate occurs chemically in the fuel stream instead. The reaction and products are the same, but the electrical current flow between the anode and cathode does not occur. The efficiency of the SOFC is thus seriously compromised.
The problem of air/fuel mixing is particularly severe when the SOFC stacks are manifolded conventionally to supply reactant gases to the interconnector grooves. The manifolds are positioned over the openings to the interconnector grooves at four sides of the stack; the ends of the stack are comprised of electrolyte plates. Here, a gas-tight seal is required between the manifolds and the stack along each of the twelve edges of the stack. Since each stack of a multi-stack SOFC must be manifolded and plumbed individually, a substantial opportunity exists for gas-leaks to occur. Further, differences in the thermal expansion coefficients between the materials used for the electrolyte plates, interconnectors, and manifolds serve to further compound the leakage problem. Thermal stress resulting from thermal expansion mismatched components can lead to leaks around the edges of the manifold and at the seams between the electrolyte plates and the interconnectors.
Attempts have been made to prevent the problem of gas-leakage between the fuel and air streams. Construction of the SOFC components from the same materials, or from materials having similar thermal expansion coefficients has decreased the possibility of a gas-leak occurring. However, the manifolds are still difficult to apply and inspect, and a separate manifold is still required for every stack. In a multi-stack system, the manifolds contribute significantly to the volume to weigh ratio of a SOFC system.
Another method of reducing gas leaks is to assemble a plurality of stacks into a module which is then manifolded for the reactant gas streams. In one arrangement, the stacks are positioned around a central air plenum that supplies air to all the adjacent stacks. Several modules may be combined to obtain greater power output, but the modules are sealed together and the assembly still has the same manifolding arrangement. Although the modular SOFC concept (see U.S. Pat. No. 5,269,902) provides some improvement in volume to weight ratio, it too has some drawbacks. The modules are difficult to construct, and can be quite large to accommodate multiple stacks. The individual modules must be sealed to one another which is difficult with large modules. Individual stacks in a module also need to be sealed simultaneously to the module framework, which becomes difficult as the number of stacks increases. Tight stack and manifold tolerances are also required to obtain a good seal of all stacks, which further contributes to the difficulties encountered in construction of SOFC modules.
Notwithstanding these and other difficulties, modular SOFC systems offer several advantages. The electrical generating capacity of a SOFC system can be increased without a corresponding increase in gas manifolding components. Several modules may be stacked together and the manifolds attached to the ends of the modular structure. Air is supplied to the central air plenum which extends through each module in the structure. Thus, a separate manifold is not required for each module in the structure. Some modular SOFC systems also facilitate series flow of fuel gases through consecutive fuel cell stacks in the modules which improves the fuel efficiency of the SOFC system.
A need exists for a compact, high-efficiency fuel cell module having improved manufacturability. The module components should be easy to manufacture and assemble using conventional methods. The module should also be compact to provide improved volume to weight ratio without sacrificing electrical generating capacity. Further, the module should provide improved sealing characteristics to avoid gas-leaks between the fuel and air streams.